Protein composition of the blood serum of horse mackerels from the Black sea, the Red sea and the Mediterranean and 5 species of the Atlantic horse mackerels from the west coast of Africa was studied by the method of paper electrophoresis. It was found that protein pattern of the blood is that of specific in each species studied. This specificity is marked either in different quantity of their protein fractions, or in their quantitative interrelation or in different features of their statistical distribution. Individual variations and sex differences in quantitative interrelation of protein fractions are considerable only during the spawning period. As for the protein composition of the blood serum, differences among subspecies within a species are less distinct than those of interspecific. Common features of protein composition of the blood serum are peculiar to all the species of horse mackerels belonging to Carangidae, that distinguishes the present family from other families in sub-order Percoidei (according to the data in literature). Material obtained on 8 closely related species of horse mackerels of the genera Trachurus and Decapterus, 5 closely related species of the genus Gobius, and numerous literary data support our previous suggestion on belonging "large" and "small" forms of the Black sea horse mackerels to one species, namely Trachurus mediterraneus.